The invention relates to a method for regulating a delivery variable of a pump, which is driven by an electric motor operated with alternating current of variable frequency, especially via a converter, wherein the input power of the motor is measured as the actual value for the delivery variable and is regulated by comparison with a desired value.
The delivery variable can be the flow rate or the pressure of the pump, although this is not measured directly.
A method of that kind is known from WO 98/04835. In that publication, the electro-motor driving the pump is an induction motor (asynchronous motor), which is driven via a frequency transformation means in the form of a converter as the adjusting element. To manage without a sensor for measuring the delivery variable, the output power or the output current of the converter and the input power or input current of the motor are measured and, by means of a table, stored in a memory, of associated current intensities (or outputs) and output frequencies of the converter, the output frequency is changed is such a way that it corresponds with the desired operating point. In this connection, it is assumed that there is a clear correlation between the measured current and the speed of the motor: if the input current of the motor rises, this indicates an increase also in the flow rate and hence a fall in pressure in the pipeline system connected to the pump. In the case of a circulating pump, however, for example, in the water circuit of a heating system, a constant pressure is desirable. The output frequency of the converter, and hence the speed of the motor, is therefore increased by the controlling system whenever the input current of the motor rises.
WO 98104835 is essentially concerned with the measurement of the electric current, but also points out that instead of the current the electric power can serve as the measured variable, without mentioning any advantages for this.
It has been demonstrated, however, that considerable control inaccuracies can occur in the delivery variable, if merely the current is measured as the value for the delivery variable. Fluctuations in the operating voltage constitute one reason for this, especially when the operating voltage is the mains voltage. If the operating voltage falls, for example, by 10%, the input power of the motor also falls by 10%. The controlling system does not register this change if just the current is being measured. The consequence is that the speed of the motor falls and the desired delivery variable of the pump is not maintained. If instead the electric input power of the motor is measured as value for the delivery variable, then changes in the operating voltage are also taken into account.
Although changes in the operating voltage as a source of error in the regulation of the delivery variable when measuring the electric input power of the motor are as a result largely excluded, it has been shown that the operating point of the electro-motor, and hence of the pump, nevertheless departs after a while from the desired operating point. The outcome can be substantial departures from the desired pressure.
The invention is based on the problem of improving a method of the kind mentioned in the introduction, without direct measurement of the pressure or flow rate of the pump, that is, without using a pressure sensor or flow rate sensor, but using the input power of the electric motor as control variable, to the effect that the desired operating point of the motor, and hence of the pump, remains stable.
According to the invention, that problem is solved in that upon a change in the temperature in the motor a corresponding compensating variable is taken into account in the control for the purposes of correcting the input power.
This solution is based on the realization that the change in the operating point is the consequence of a change in the ohmic resistance in the stator and rotor of the motor. This change is in turn primarily the consequence of heating of the motor through electrical losses or convective heat, for example, through hot water that is being conveyed by the pump. The electrical losses in the motor, and hence the slip, therefore increase, so that the output power of the motor, its speed and hence also the pressure of the pump, decrease. Heating of the motor therefore has two effects: firstly, losses in the rotor are increased, with the result that less power is delivered to the shaft. With regulation of the input power of the motor in which only the input power is measured, this power loss is not detected. In the case of such regulation, there would therefore be no compensation for the power loss in the rotor. A second effect of heating of the motor is that its slip increases. This means that less power is transferred to the rotor. The input power regulation interprets this erroneously as reduced power requirement and reduces the operating frequency of the motor. The operating point of the pump therefore differs from the desired operating point. The invention compensates for the temperature-dependent pressure fall, without the pressure being measured directly.
This can be achieved in an especially simple manner in that the compensating variable is retrieved up from an empirically prepared, stored table of associated input power change values and temperature values of the motor in dependence on the operating temperature of the motor.
Another possibility is that a table that contains the pressure change of the pump at different input powers of the motor at the time the operating temperature of the motor is reached is empirically prepared and stored, and from the table a pressure change is retrieved in dependence on the actual value of the input power as a compensating variable in the regulation.
A somewhat more accurate solution consists in that from the compensating variable and a frequency control variable an approximate actual speed value is calculated, which, together with a desired pressure value, is used to retrieve an accompanying desired input power value from a stored, empirically prepared table of associated values of input power and speed of the motor.
Another solution to the problem posed consists in accordance with the invention in that associated values of the input power and the speed of the motor at a predetermined desired pressure value are determined empirically and stored as a table, and that during operation the value of the input power belonging to a measured or approximately calculated speed of the motor is retrieved from the table as desired value for regulating the input power. In the case of this solution, previous measurement of the dependency of the pump pressure on the motor temperature is not needed, because the speed of the motor or of the pump is directly measured or approximately calculated and a temperature-dependent change in the output power of the motor is used for compensation.